Hard for me to say because it’s been a while since I sold the SP2K, but my general feeling is that SP3K is a wholesale upgrade on SP2K for sound quality. Blacker background, more detailed, more refined. Tonally I think SP3K has a slightly bassier, musical sound but again it’s hard to say with any certainty without hearing them side-by-side.
I had a friends SP2KT unit briefly last week so I got to compare it directly with SP3K, and it was obvious to me that SP3K was on another level sonically, and that was when SP3K only had a few hours use. I remember when I compared SP2K with SP2KT and I also thought SP2K sounded better than SP2KT at that time, but the gap wasn’t as big as it was between SP3K and SP2KT.